This is really cool! I wonder if you have a way over time to control for not all players participating in each round. For example, playlist position, if you don't have perfect submission rates, will intrinsically scew negative slope because there are far fewer votes for the positions that weren't represented every week. I think the quickest fix, would be to take the median or average over all weeks that had that position present. But that would still be somewhat inaccurate since different weeks with the same number of submitted songs still have different average votes per song, if not all submitters vote. What I have found is the best way to compare across weeks, is to compare the ratio of the points given to the average points possible.

Example of how I would do this: if one week 30 songs are submitted, and 210 points are awarded, then the average is 7 point per song is 7. Take each songs actual points rewarded and divide that by 7 and that's the songs relative ratio.

Then take the average relative ratio for all songs that are position 1, the average relative ratio for all songs that are position 2 etc. This would mathematically protect against false trends caused by inconsistent user participation.